W9WDX

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I have performed the negative peak compression for a Cobra 2x family, or any other transformer coupled radio.

Total cost, about a dollar. Switchable. I will be drawing schematics up later for a switched one (just set and forget), and a variable NPC that will allow you to dial your carrier level to whatever you want to set it at, for NEGATIVE peaks... Positive peaks will remain unchanged.

Have it working in a Cobra 29 Nightwatch. VERY simple install. Completing the first circuit took about 4 hours. I made a second circuit in 5 minutes.

Incidentally, this IS a circuit that can be built in 'spare time'. It WILL work in ANY radio, but I DESIGNED it for transformer coupled AM transmitters (meaning, you can use this in a tubed Tram, Browning, etc... ANY high level modulated radio this circuit will work in.

Simple? Oh yeah. The first one was a single switch (and you don't need THAT, unless you want a switchable bleedover circuit lol) and 4 wires. Solder tack to the power switch, one to ground, and two to the PC board. THAT'S IT.

I have performed the negative peak compression for a Cobra 2x family, or any other transformer coupled radio.

Total cost, about a dollar. Switchable. I will be drawing schematics up later for a switched one (just set and forget), and a variable NPC that will allow you to dial your carrier level to whatever you want to set it at, for NEGATIVE peaks... Positive peaks will remain unchanged.

Have it working in a Cobra 29 Nightwatch. VERY simple install. Completing the first circuit took about 4 hours. I made a second circuit in 5 minutes.

Incidentally, this IS a circuit that can be built in 'spare time'. It WILL work in ANY radio, but I DESIGNED it for transformer coupled AM transmitters (meaning, you can use this in a tubed Tram, Browning, etc... ANY high level modulated radio this circuit will work in.

Simple? Oh yeah. The first one was a single switch (and you don't need THAT, unless you want a switchable bleedover circuit lol) and 4 wires. Solder tack to the power switch, one to ground, and two to the PC board. THAT'S IT.

OK, here's the 'final' circuit. This is for the 'standard' model, no variable to mess with.

The unlabeled diode is the existing 'anti spike' diode all radios have coming from the modulation transformer. You wrap the two other diodes around it.

This is the way the circuit works.

The diodes and resistors form a divider that allows the D2 to ONLY conduct during NEGATIVE mod peaks.... The HARDER the negative mod peak, the HARDER the diode conducts. During this conduction, it allows B+ (tube speak... Same as "the hot lead" in 12 volt stuff) from the resistor and capacitor to conduct through D2. By varying the resistance of the approximately 50 ohm resistor, you can vary the amount of negative peak limiting that the circuit provides.

I did NOT invent this... This is an adaptation of different peak limiters.

This circuit does NOT work 'just like npc' on the 148 or other class A series limiter radios.... IE, it DOES work at an audio rate, but it will NOT prevent negative 'peaks'... It prevents the carrier from being COMPLETELY pinched off. THIS is what causes the MAJORITY of splatter and garbage. This circuit will NOT clean up the waveform... I can add a splatter filter (to clean up the pos and neg peaks), but those add distortion of their own.

Also, the approximate 50 ohm resistor. It will run COLD if you don't 'bang on the limiter', so to speak. The MORE negative peaks you add (ie, the HOTTER you run the audio gain / power mic / etc), the HARDER this resistor has to conduct..... With a stock mic, a pair of 1/2 watt resistors in parallel (100 ohms) can be held in the hand. With the 'typical' levels a power mic produces, they will get WARM....

FOR THOSE THAT WISH TO MODIFY THE CIRCUIT!!!

If you want variable negative peak limiting, then take the "approximate" 50 ohm resistor and replace it with a simple LM317 or other 'variable' regulator. Copy any typical schematic for the voltage regulator.....

The 5.6 ohm resistor I made with a pair of 10 ohm half watts. The total package needs to be able to pass 1/20th of the total AUDIO power fed to the final... In a typical CB, this means it needs to pass ABOUT a half watt, average. The diodes need to be able to stand 4 times the unmodulated input voltage (13.8 on a transistor rig).

IF YOU WANT TO DO THIS TO A DIFFERENT 'STYLE' CHASSIS, here is the math:

The diodes are 1/2 the final plate impedance.... IE, (Voltage x voltage) / (2 X Power out) and take that answer and divide it by two. For a 50 watt MOSFET, it would work like this:

13X13 = 169. 169 / 100 = 1.69 ohms. Half that. .8 ohms. This is the MATHEMATICAL way to find the resistor value. once it is found, you can place a RHEOSTAT or pot there to vary it to get the level you want. Be careful, in tube stuff, full B+ can be across this resistor, depending on mod index. You can use the same LM317 to feed the circuit..... Tubed or transistor, you only need a few volts to ensure the carrier is NEVER pinched off.

Want to put this in a series modulated radio? Like the 148 / etc? Do the standard NPC volt the final mod, and the wire that goes to the driver, you cut. Insert a "anti spike diode" in series, and otherwise wire up the circuit above. Is it easier than a cap? No. BUT, if you have a buttload of these made up, it makes it easier, and if you put a variable power supply on it, you have a LOT better method of keepalive than NPC...

Incidentally, the power supply that feeds the keepalive circuit MUST be low impedance. Therefore, even though most other schematics that are similiar to this show it with the cap being unnecessary, I keep mine in circuit ALL the time.... Low ESR cap means you get a low R power supply.

Enjoy. Any questions, please feel free to ask. Simplest circuit I've come up with , and this will do all that is needed.

A video of what, exactly? I can see making a 'how to' if it was some esoteric or hard to source parts, but a pair of diodes and resistors, as well as a cap is about as simplistic as can be....

If need be, and I knew what people WANTED a video of, I could do one... But this is fairly self explanatory: I'm betting people are going WTF, this doesn't hook up like NPC on the electronically regulated chassis..... And blowing it off.

Circuits like this seperate screwdriver wizards from technicians.... Unfortunately for America, the math involved will stump some.....

Scope readings? This circuit prevents 'baselining' the audio, or in other words, prevents the carrier from EVER reaching 0 volts, cut off, etc.

Anywho.. Not against making a video, just don't know what people want a video of?????

The diodes and resistors form a divider that allows the D2 to ONLY conduct during NEGATIVE mod peaks.... The HARDER the negative mod peak, the HARDER the diode conducts. During this conduction, it allows B+ (tube speak... Same as "the hot lead" in 12 volt stuff) from the resistor and capacitor to conduct through D2. By varying the resistance of the approximately 50 ohm resistor, you can vary the amount of negative peak limiting that the circuit provides.

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TOLL_FREE -

Your circuit may work (I haven't tried it yet), but I don't understand your explanation. The output from the transformer has a DC (13 volts) and an AC (Audio) component. Since D1 and D2 are reverse biased with respect to the DC, it is unaffected. But BOTH D1 and D2 are forward biased with respect to negative audio peaks and both will conduct on negative peaks. However, I think that D1 will conduct more than D2 because of the voltage drop across the Anti-spike diode. This is confusing to me. Please explain the function of D1 in this circuit. Thanks. 73s.

If I had a virgin 29, I'd do this and make a video showing how/where to do it. It looks pretty straight forward. Thanks for this!

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Wish I still had my 29, I would send it to ya for a sample. All I got here is a 146GTL which I did a Diode/Res NPC mod to it. Does well @ 1 watt DK/Swinging full 12+ watts. If ya ever come across a 29 post up a vid of the mod.

The amplitude of the carrier in an AM signal is the same with or without modulation. Looking at the familiar "rf envelope" display on a scope is misleading because it can't display the carrier and the sidebands seperately. It only displays the vector sum of the three signals added together. If you look at an am signal on a spectrum analyzer it shows that the carrier amplitude is the same with or without modulation.

The amplitude of the carrier in an AM signal is the same with or without modulation. Looking at the familiar "rf envelope" display on a scope is misleading because it can't display the carrier and the sidebands seperately. It only displays the vector sum of the three signals added together. If you look at an am signal on a spectrum analyzer it shows that the carrier amplitude is the same with or without modulation.

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I own a spec an as well as an oscope. What you are saying is textbook, and WILL be what is reproduced WHEN YOU DO NOT EXCEED 100 PERCENT MODULATION in a high level modulated system.

When you OVERMODULATE in the NEGATIVE direction, you actually cut voltage off COMPLETELY to the modulated stage(s).... In effect, BASELINING the modulator... This is what actually causes splatter, IMD, etc. You CAN see this effect on a scope, this is the little bar you get between modulated waveforms (the valleys being extended between peaks, causing actual space between the individual RF cycles.... THIS is 0 watts of carrier, THIS is splatter, AND when you look at BOTH the Spec An AND the Oscope you WILL see this effect.)

What the circuit I presented does is PREVENT the modulator from EVER hitting 0 watts, 0 volts, etc.... However you want to PUT it. It does NOTHING to the modulated waveform (at least, that I can detect), it ONLY keeps a couple volts on the finals AT ALL TIMES, whereas the MODULATED waveform, when modulated in EXCESS of 100 percent will actually CUT ALL VOLTAGE off to the final and driver... PLEASE explain to me how you will still have carrier, when you have 0 volts being delivered to the collector / plate of the amplifying device, to put it in other terms.

If your limiters are STILL intact, and act upon the NEGATIVE going modulated voltage, then YOU ARE CORRECT, and you will NEVER baseline the audio.

HENCE the reason I stated THIS circuit will work on any HIGH LEVEL modulated radio. A low level modulated radio should still use the Eitner induced NPC, which was borrowed and modified version of the BDW CBDoc NPC. The low level modulated radios (balanced mods) will actually take the negative going waveform and 'flip' it, causing the carrier to never baseline, but introducing the 'overmodulated' sound that the lincoln, 2510, jackson, etc have, versus the "louder than hell" overmod sound that the high level radios have (which is actually nothing more than IMD....... Baselining the carrier, etc).

THIS is the type of discussion I hoped to implement... In a PROPERLY functioning AM HIGH LEVEL transmitter, that is NOT overmodulated to the NEGATIVE direction, you're right, the carrier won't be cut off... HOWEVER, if you exceed 100 percent modulation, the spec an, the oscope and a fast acting voltmeter with peak and hold detection will show it actually being cut off....

Also, the resistor that feeds this circuit will get noticeably warm under use. If you HAMMER the negative peaks, it will get HOT. Easy way to tell if your overdriving your modulator is to test the heat on the resistor... If you do the math correctly, it will run barely warm...... PROVIDED you're not running a Silver Eagle mic at half throttle, limiters ripped out and dynamic wide open! At that point, you're pretty much baselining EVERY half cycle, and the resistor should be sized in wattage at HALF your total modulator power, or more.

As to LC's question about carrier: You'd want to put MORE diodes in the series diode, ah la the old "dial-a-watt" methods from Secret CB. Forget about the jumper method. The problem with the jumper and then going to the RC circuit is this: You will NEVER get full drive to the final amplifier with the driver at less than peak output! Then, on modulated peaks, you end up with a slight ripple or valley at the CREST of the mod peak. I'd lower the final AND driver Vcc with a string of diodes, use the baseline preventer here, and you're ALWAYS looking at the proper input and output impedances from the modulator to the driver to the final. I've also played with modulating the pre-driver, but didn't get anywhere near the effect I wanted.... Tried hitting the pre-driver with a variable voltage on the collector, and after losing about 2 watts of carrier (8 watts was where it was when I started), the waveform on the output went all to hell. On the scope and analyzer, the best method thus far is dial a watt.... The 3 watts you lose PEP won't be noticed.

The thing to watch about ripping the limiter out is this: You can STILL overdrive the modulator stage (the audio chip) causing a triangle waveform on it's output. The transformer is GIGO, if you hit it with a non-linear waveform on it's input, the transformer will only amplify it and send it to the finals..... Watching the modulating waveform on channel A and the output on Channel B will let you know exactly how much forward power you can have..... Hint, only HALF the modulator power can go forward... If you want more, you require HARDWARE in your audio chain to actually increase the AVERAGE power input to the radio.

With careful tuning of the radios output section, and a 1969, THIS is the way to get 75 PEP out of a 29.

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